Flux-gate magnetometers have been used for some time to measure the earth's magnetic field. They have been very instrumental in high accuracy surveying of oil wells or boreholes. A flux-gate consists of a core of highly magnetically permeable material, such as Permalloy-80. Such material saturates at low field strengths and has a magnetic characteristic curve (i.e., BHH curve or hysteresis curve) exhibiting a high degree of "squareness". (See U.S. Pat. No. 4,447,776.) The core is wrapped by an electrical winding through which an alternating current is passed, so as to create an oscillating, saturating field. One or more sensing coils are wrapped around the core to measure and balance the saturating field. This Lechnology is well know and has been used with some success for many years. Some representative U.S. Pat. Nos. are: 3,403,329; 3,982,431; 4,013,945; and 4,181,014.
The core material is highly sensitive to stress and strain. Therefore, any useful flux-gate magnetometer, especially a flux-gate sensor used in an oil drilling environment, has to have its core mounted to withstand the stress caused by shock, vibration, pressure and temperature. To this end, the core material is conventionally wrapped around a bobbin or spool to stabilize the material and form a base or frame around which the coils may be wrapped.
In some products, the bobbin form is made from a ceramic substrate. (See U.S. Pat. No. 3,873,913) A ceramic has several advantages: it is non-magnetic to a high degree, it is stable over annealing temperatures of 1500 degrees F. and, it is capable of performing in a rugged environment. However, there are some disadvantages: the thermal coefficient of expansion is significantly different from that of the core material, often resulting in a loose fit of the core at high temperatures; stress induced property changes are possible, it is difficult to epoxy bond to the core material wound around it; and it is relatively large in size.
Some bobbins are made of stainless steel. See U.S. Pat. Nos. 4,068,164 and 4,447,776). A stainless steel bobbin overcomes some of the drawbacks of a ceramic bobbin. In particular, stainless steel has a temperature coefficient relatively close to that of the core material. Moreover, one and of the core material can be welded to the bobbin for a good start in winding the core material (usually a thin, foil-like tape) about the bobbin. Moreover; it has a relatively small size, and stainless steel can withstand the high temperatures of annealing and the extreme temperature environment of a borehole application.
Heretofore, it was thought that by proper selection and handling, ordinary austenitic stainless steel is sufficiently non-magnetic so as not to affect the performance of a flux-gate. As a result of testing and calibration of high sensitivity flux-gates, I determined that ordinary austenitic stainless steels do affect the performance of a flux-gate. In particular, I have found that the residual permeability of a stainless steel bobbin causes a bias shift, dependent on the strength of the external field and the amount of permeability.
My invention offers a solution to these problems without introducing any of the drawbacks of a ceramic bobbin. Specifically, I propose a bobbin construction which has ultra-low permeability, is weldable through normal methods, withstands the temperature of annealing, is rugged to shock and vibration, is relatively small in size, approaching that of stainless steel, and has a temperature coefficient comparable to the core material.